Cardiac output is a very important parameter for evaluation of total cardiac performance in a patient. Measurement of this parameter is a valuable tool in clinical practice as well as in research. Measurements are normally performed on critically ill patients or during major heart surgery. In patients who require pharmacologic support with potent vasoactive drugs, measurement of cardiac output is considered to be necessary.
It is well known to measure cardiac output in patients using an indicator dilution technique. In this technique, a known amount of dye, or a known amount of a cold liquid in the form of a cold saline bolus, for example, is injected into the patient's circulation. This is preferably done into the right atrium. An indicator dilution curve is then measured downstream in the pulmonary artery or the aorta.
Using this technique, cardiac output can be calculated from the area under the indicator dilution curve and the amount of the indicator. The indicator dilution curve is a dye concentration curve or a temperature curve, depending upon whether a dye or a cold liquid was injected. However, this method for measuring cardiac output requires assessment of indicator curves by invasive measurement through a catheter in the pulmonary artery or aorta. Because the use of invasive catheters is associated with significant risk, measurement of cardiac output in this manner is confined to critically ill patients in which the risk-benefit ratio is considered to be reasonable.
There have been attempts to measure an indicator dilution curve using an indocyanine green dye non-invasively. However, these attempts differ significantly from the present invention. Essentially, the previous attempts have tried to quantify an absolute concentration of intravascular indocyanine green dye concentration by direct measurement of light transmission or reflection. However, this did not produce accurate results because the Lambert-Beer law is not valid for light transmitted through tissues because multiple light scattering occurs. In these attempts the pulsatile nature of the lights signal was a handicap to the measurement and was generally filtered out.